An Ultrabook is a higher-end type of subnotebook defined by Intel.[1] Intel claims a trademark in the name Ultrabook.[2]
Ultrabooks are designed to feature reduced size and weight and long
battery life while retaining strong performance. They use low-power
Intel CULV processors with integrated graphics, solid-state drives for fast loading times, and unibody chassis to fit larger batteries into smaller cases.[3] Because of their minimal size, the ability to have many ports (USB, HDMI, VGA, ethernet, etc) is limited.

By this marketing initiative and an associated $300 million fund,
Intel hopes to influence the slumping PC market against rising
competition from tablet computers, which are typically powered by competing ARM-based processors.[4]

Larger Screen Sizes

Though not a "requirement" per se, more 14 and 15" screens can be
expected later on into 2012. At a presentation at the Consumer
Electronics Show, an Intel Manager stated that market analysis revealed
that screen size motivated some of the reluctance to switch to 13"
Ultrabooks. As a result, Intel is planning to ensure, through
cooperation with manufacturers, a 14 or 15-inch screen on 50% of the 75
Ultrabooks that will likely come to market in 2012. [10][11]

Third phase (2013)

List of models

Because of the novelty of the Ultrabook, many manufacturers been
unable to meet all of Intel's requirements in their initial models.
These deviations (especially cost overruns) are expected to decline as
manufacturing processes for Ultrabooks mature. Sections in bold indicate
an area where the model fails to meet Ultrabook requirements.

Notes:1 Only base laptops are shown here.2 Pricing announced (in USD and on the US market, unless specified) by manufacturer or at time of product release.3 Some Ultrabooks have a uniform Thickness.4 These models include two drives. The SDD is pre-configured to store only hibernation data (for faster S4 resume).